The invention relates to a process for operating an open ring-type furnace, with firing shafts partly grouped together in the fire reversal unit, for manufacturing shaped carbonaceous bodies, mainly electrodes for the aluminum fused salt electrolytic process, by regulating at least the temperature and the negative pressure, and relates too to a device for carrying out the process.
The open ring-type furnace comprises a number of stationary baking chambers or pits aligned in rows next to each other and divided off by walls running transverse to the direction of progress of the fire, and by firing shafts arranged in cassettes and running in the direction of progress of the fire. A ring arrangement of the firing shafts is achieved by parallel arrangement of two rows or baking chambers, one on each side of the furnace, and by connecting up the two fire shaft systems at the ends of these rows via fire reversal units. The fire shaft systems are either connected up in such a manner that all firing shafts join up to a common duct, or are partly grouped together at the ends of the two rows in the fire reversal unit. In the case of eight firing shafts, for example, the three inner firing shafts and the two outer firing shafts are grouped together and connected to the corresponding groups on the other side of the furnace. It is of course also possible to connect up the fire shafts individually. This is, however, not advantageous as it would involve a very complicated construction requiring a great deal of space as well as yielding a low degree of firing efficiency.
The mode of operation of such baking furnaces has an extremely important influence on the quality of anode produced. Both the rate of heating to the baking temperature and the average baking temperature have a decisive influence on anode quality, in particular with regard to strength, electrical conductivity and reactivity. If temperature changes occur too quickly, the anodes may develop cracks or, if the average baking temperature is too low their reactivity may be poor.
In view of this there has been no lack of attempts to automate, at least in part, the operation of baking furnaces by means of process control. For this one needs an appropriate operating plan or model in order that the temperature change during the progress of a fire can be kept under control.
The operating model is itself influenced by the starting parameters, the planned temperature sequences and gradients, and by the correction plan on which the model is based. The correction plan indicates where a particular parameter is to be left unaffected or how it is to be changed in order to prevent over or undershooting the intended temperature or to make correction. Microprocessors are employed to regulate the appropriate parameter-mainly the temperature in the fire shaft and the negative pressure in the suction unit. These register the data and issue the commands for correction to the appropriate adjustment means-valves on the burners and slides on the suction unit. By starting parameters is to be understood the external conditions which are of fundamental importance e.g. the type of furnace construction and the condition of the furnace. Of great importance in this respect is the construction of the fire reversal unit.
When establishing the operating plan or model one usually obtains empirically determined relationships between the temperature change in the furnace and the properties of the anodes which experienced that change. Those conditions which lead to above average anode quality are then regarded as optimal. In subsequent runs with the furnace the aim is then to approach as closely as possible the advantageous sequence of baking temperatures using the adjustment means on the furnace.
When the fire is wholly on one side of the furnace it is possible, without any difficulty using the present day operating model, to regulate each firing shaft individually such that the planned sequence of baking temperatures is followed.
For reasons concerned with furnace design, however, the ring-type furnace can not be regulated by the same principle during the complete traverse of a fire round the furnace i.e. during the progress of a fire through all chambers. If a fire is at a stage where it is particularly on one side of the furnace and partly on the other side i.e. in the phase when the fire reverses its direction by moving round the end of one row of chambers to the next row on the other side, and the fire reversal unit is included in the fire then, due to the combination or termination of the fire shafts on the downstream side and the allocation of the fire shafts on the upstream side, it is no longer possible to achieve satisfactory individual adjustment of the fire shaft temperatures by regulating the negative pressure in the individual fire shafts on the side of the furnace where the leading end of the fire i.e. the first pre-heating chamber is situated. The result in such a case is a drop in anode quality.
The object of the present invention is therefore to operate a ring-type furnace of the kind described above-both without but in particular with process control-in such a manner that the quality of the anodes does not depend on the position of the anodes in the furnace. A particular object of the invention is to employ a means of process control which overcomes the diadvantages suffered during the operation of the furnace at the phase where fire reversal takes place.